The present invention relates to a tire building drum.
Tires comprising a carcass with two bead bundles are normally formed on a building drum, also known as a tire forming drum, comprising a cylindrical central body, and two half-drums located on opposite sides of the central body and moved in opposite directions along a common axis by a central actuating device. Each half-drum comprises an expandable bead clamping device normally defined by a ring of sectors, which are movable, by a normally pneumatic actuator, substantially radially with respect to the drum axis and to and from an expanded position clamping a respective bead bundle in a given axial position along the half-drum and on the outside of a carcass, of a tire being formed, lying in an initially flat position on the building drum.
Each half-drum also comprises at least one shaping bladder which lies flat at rest on the half-drum, between the respective bead clamping device and the central body, and is inflatable to shape the carcass; and at least one turn-up bladder which lies flat at rest on the half-drum on the opposite side of the bead clamping device with respect to the shaping bladder, and is inflatable to turn a lateral portion of a body ply of the tire being formed outwardly up about the respective bead bundle.
Each turn-up bladder of building drums of the above type normally comprises an annular lip, which extends towards the respective shaping bladder to cover the sectors of the respective bead clamping device while leaving the entire outer surface of the shaping bladder clear. The turn-up bladder and provides, among other things, for preventing the expanding sectors of the respective bead clamping device from xe2x80x9cmarkingxe2x80x9d the surface of the coating skim of the carcass, and for keeping the respective turn-up bladder as close as possible to the respective bead clamping device when shaping the carcass.
A major drawback of known building drums of the above type lies in each shaping bladder necessarily sliding, when shaping the carcass, in contact with the coating skim of the carcass. Being still green, the coating skim would be ruined by adhering to the outer surfaces of the shaping bladders if it were not lubricated with oil, which is not only expensive but also takes a relatively long time to apply and creates difficulties at the curing stage.
Moreover, for known forming drums of the above type, because of the relatively small transverse dimension of each annular lip and the oil applied to the coating skim of the carcass, the annular lips will tend to separate from the bead clamping devices. From the sectors of the respective bead clamping device, the coating skim of the carcass is left unprotected and, more importantly, the respective bead bundle is thereby free to move axially with respect to the body ply.
One aspect of the present invention is to provide a building drum designed to eliminate the aforementioned drawbacks.
According to the present invention, there is provided a drum for building tires, the drum comprising two half-drums, each of which comprises a substantially cylindrical outer tubular body, a bead clamping device expandable through said tubular body, at least one shaping bladder, first fastening means for connecting said shaping bladder to said tubular body, a turn-up bladder having an outer annular lip extending towards the respective shaping bladder, and second fastening means for connecting said turn-up bladder to said tubular body on the opposite side of the respective bead clamping device with respect to said shaping bladder; wherein each said annular lip extends, at rest, along the respective half-drum to cover the respective said bead clamping device and a substantial portion of the respective shaping bladder.
In a preferred embodiment of the above building drum, said lip is of such adequate width as to cover, at rest, substantially the whole respective said shaping bladder.